Magnetic recording medium having narrow pulse width characteristics

ABSTRACT

A dual-layer magnetic recording medium including a non-magnetic substrate having a front side and a back side, a lower support layer formed over the front side and a magnetic upper recording layer formed over the lower layer, wherein the magnetic layer contains a volume concentration of at least about 35% of a magnetic metallic particulate pigment having a coercivity of at least about 2000 Oersteds (Oe), and a binder system for the pigment. The magnetic recording medium exhibits an orientation ratio greater than 2.0, a pulsewidth (PW50) value of less than about 500 nm, and a remanence-thickness product, Mr*t, of less than about 5.0 memu/cm 2 .

THE FIELD OF THE INVENTION

[0001] The present invention relates to magnetic recording media such asmagnetic tapes, and more specifically to the magnetic layer of the mediawhich contains particulate metallic pigments that have high magneticcoercivity and a high volume concentration. The media exhibit narrowerpulsewidth characteristics and lowered remanence-thickness product.

BACKGROUND OF THE INVENTION

[0002] Magnetic recording media are widely used in audio tapes, videotapes, computer tapes, disks and the like. Magnetic media may use thinmetal layers as the recording layers, or may comprise particulatemagnetic compounds as the recording layer. The latter type of recordingmedia employs particulate materials such as ferromagnetic iron oxides,chromium oxides, ferromagnetic alloy powders and the like dispersed inbinders and coated on a substrate. In general terms, magnetic recordingmedia generally comprise a magnetic layer coated onto at least one sideof a non-magnetic substrate (e.g., a film for magnetic recording tapeapplications).

[0003] In certain designs, the magnetic coating (or “front coating”) isformed as a single layer directly onto a non-magnetic substrate. In aneffort to reduce the thickness of this magnetic recording layer, analternative approach has been developed to form the front coating as adual-layer construction, including a support layer (or “lower layer”) onthe substrate and a reduced-thickness magnetic layer (or “upper layer”)formed directly on the support or lower layer. With this construction,the lower layer is typically non-magnetic or substantially non-magnetic,generally comprised of a non-magnetic powder and a binder. Conversely,the upper layer comprises a magnetic metal particle powder or pigmentdispersed in a polymeric binder.

[0004] In addition, with magnetic recording tapes, a backside coating istypically applied to the opposing side of the non-magnetic substrate inorder to improve the durability, conductivity, and trackingcharacteristics of the media.

[0005] It is also known in the art to calender the medium during itsmanufacture, e.g., to pass the medium through a series of opposedrollers before winding it into a roll, to improve surface smoothness. Itis also known to heat-soak magnetic tape in wound form, after thecoating and calendering processes, to “cure” the tape's coatings andincrease the glass transition temperatures of the binder matrices. Afterthe curing is complete, the tape is converted for use in cartridges.Calendering occurs at a calendering temperature of, for example, betweenabout 90° C. and about 95° C. The calendering includes passing thesubstrate between opposed, generally non-compliant rolls, and optionallyfurther includes calendering the substrate between additional opposedrolls, at least one of the additional opposed rolls being generallycompliant. The calendering includes off-line calendering, andadditionally includes in-line calendering, using at least one generallycompliant roll, prior to the heat-curing, according to embodiments ofthe invention.

[0006] The single layer coating on magnetic recording media, and bothlayers of dual-layer magnetic recording media, generally include agranular pigment. Popular pigments are metal oxides, ferrimagnetic orferromagnetic metal oxides, and ferromagnetic metal alloys; the materialin the lower layer of the dual-layer media is generally non-magnetic,and that in the upper layer is magnetic. Different pigments havedifferent surface properties; the metal particles often have a stronglybasic surface. Recording media often utilize alpha iron oxide (α-Fe₂O₃)particles in the formulations; dual-layer recording media may utilizesuch particles in the nonmagnetic lower layer formulations, along withcarbon black particles. The magnetic layer of such recording media oftenutilize gamma iron oxide (γ-Fe₂O₃), magnetite (Fe₃O₄), cobalt-doped ironoxides, or ferromagnetic metal or metal alloy powders, along with carbonblack particles.

[0007] All front coatings or layers of magnetic recording mediagenerally include a binder composition. The binder composition performssuch functions as dispersing the particulate materials, increasingadhesion between layers and to the substrate, imparting cohesion of theparticles in the layers, improving gloss and the like. As might beexpected, the formulation specifics associated with the requisite upperlayer, lower layer, and back coat, as well as coating of the same to anappropriate substrate are highly complex, and vary from manufacturer tomanufacturer; however, most binders include such materials asthermoplastic resins. Many factors affect the performance of magneticmedia, including the binder system; the lubricants; the method offorming a dispersion from the ingredients; the coating, drying andcalendering conditions; the level of cleanliness around the coating headand calendering rolls; the smoothness of the tape; the number,frequency, and heights of protuberances on the magnetic surface. Onemeasure of magnetic media performance is pulsewidth, often abbreviatedas PW50. PW50 is a measurement of a signal recorded at such a lowdensity that the transitions are isolated from one another; i.e., theydo not interact or interfere with one another. The amplified,unequalized and unfiltered signal from the read head is displayed on anoscilloscope and the width along the time axis of the resulting positiveand/or negative pulses halfway from the baseline to their peaks ismeasured. This time interval is multiplied by the tape transport speedto obtain the pulsewidth, as a distance.

[0008] It has now been discovered that using magnetic recording mediahaving multiple layers wherein the upper magnetic layer contains certainmetallic pigments in the magnetic layer of a magnetic recording medium,e.g., particle pigments having a coercivity of greater than about 2000Oersteds (Oe), with particles having lengths of less than about 100nanometers (nm), preferably less than 80 nm at a volume concentration ofgreater than about 35%, significantly narrows the PW50 characteristicsof the resulting medium.

SUMMARY OF THE INVENTION

[0009] One aspect of the invention provides a dual-layer magneticrecording medium including a non-magnetic substrate, a lower supportlayer, and a magnetic upper layer, and optionally, a back coat. Thesubstrate defines a front side and back side, with the back coat, ifdesired, being formed on the back side. The magnetic upper layer isdisposed over the lower support layer on the front side of the substrateand includes a volume concentration of at least about 35% of a primarymagnetic metallic particulate pigment material having a coercivity of atleast about 2000 Oe, and an average particle size of less than about 100nm, and a binder system therefor. The coated medium preferably has anorientation ratio greater than about 2.0, a PW50 of less than about 500nm, and a remanence-thickness product, Mr*t, of less than about 5.0memu/cm², preferably less than about 4.0 memu/cm².

[0010] Another aspect of the invention provides a dual-layer magneticrecording medium comprising a non-magnetic substrate having a front sideand a back side, a lower support layer and an upper magnetic recordinglayer formed on the front side, said upper magnetic layer including avolume concentration of at least about 40% of a primary magneticmetallic particulate pigment material having a coercivity of at leastabout 2000 Oe, and an average particle size of less than about 100 nm,wherein the coated medium has an orientation ratio greater than about2.0, a PW50 of less than about 500 nm, and a remanence-thicknessproduct, Mr*t, of less than about 5.0 memu/cm², preferably less thanabout 4.0 memu/cm².

[0011] As used herein, all weights, ratios and amounts are by weightunless otherwise specified.

[0012] As used herein, the following terms have these meanings:

[0013] 1. The term “low molecular weight” means having a molecularweight of less than about 500.

[0014] 2. The terms “coercivity” and “magnetic coercivity” aresynonymous, are abbreviated (Hc), and refer to the intensity of themagnetic field needed to reduce the magnetization of a ferromagneticmaterial to zero after it has reached saturation.

[0015] 3. The term “Oersted” refers to a unit of magnetic field and isequivalent to (¼π) kA/m.

[0016] 4. The term “soft” means magnetically soft, i.e., having acoercivity of less than about 300 Oersteds (Oe).

[0017] 5. The terms “layer” and “coating” are used interchangeably torefer to a coated composition.

[0018] 6. The terms “PW50” and “pulsewidth” are used interchangeablyherein. Pulsewidth is tested by recording a signal on a magneticrecording medium at a sufficiently low density that the transitions areisolated from one another; i.e., they do not interact or interfere withone another. The amplified, unequalized and unfiltered signal from theread head is displayed on an oscilloscope and the width along the timeaxis of the resulting positive and/or negative pulses halfway from thebaseline to their peaks is measured. This time interval is multiplied bythe tape transport speed to obtain the pulsewidth, as a distance.

[0019] 7. The term “remanence-thickness product,” is abbreviated Mr*t,and means the product of the remanent magnetization after saturation ina strong magnetic field (10 kOe) multiplied by the thickness of themagnetic coating. This value is measured in memu/cm².

[0020] 8. The term “orientation ratio” means the ratio of the remanentmagnetization (at zero applied magnetic field) after saturation in astrong magnetic field (10 kOe) measured in the direction parallel tothat of the recording medium's intended transport to the correspondingquantity measured in the direction transverse (perpendicular, but in theplane of the medium) to that of the recording medium's intendedtransport.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The following detailed description describes certain embodimentsand is not to be taken in a limiting sense. The scope of the presentinvention is defined by the appended claims. The magnetic recordingmedium includes a non-magnetic substrate, a magnetic upper layer, alower support layer, or sublayer, and, typically, a back coat layer. Thevarious components are described in greater detail below. In generalterms, however, the magnetic upper layer includes a primary metallicpigment powder having a coercivity of greater than about 2000 Oe, and abinder system for the pigment. The lower support layer includes aprimary powder material consisting of particles that are essentiallynonmagnetic (although some magnetic particles having a coercivity of 300Oe or less may be included) dispersed in a binder system.

[0022] In accordance with the current invention, coated magneticrecording media have an orientation ratio of greater than about 2.0,preferably greater than about 2.2. The medium exhibits a narrowed PW50,i.e., the PW50 is less than about 500 nm, preferably less than about 450nm

[0023] The Magnetic Recording Layer

[0024] The upper layer of the medium is a magnetic recording layer. Themagnetic recording layer has a thickness of from about 2 microinches(0.05 μm) to about 20 microinches (0.50 μm) in thickness, preferablyfrom about 2 to about 15 microinches. The medium also has aremanence-thickness product, Mr*t, a thickness measurement, of less thanabout 5.0 memu/cm², preferably less than 4.0 memu/cm².

[0025] The magnetic metal particle pigment comprises a primary magneticmetal particle pigment having a coercivity of at least about 2000 Oe,preferably at least about 2300 Oe. Such pigment preferably has anaverage particle length of less than about 100 nm, preferably less thanabout 80 nm. The pigment is present in the upper magnetic layer in avolume concentration of at least about 35%, preferably about 40%.

[0026] The magnetic metal particle pigments have a compositionincluding, but not limited to, metallic iron and/or alloys of iron withcobalt and/or nickel, and magnetic or non-magnetic oxides of iron, otherelements, or mixtures thereof. Alternatively, the magnetic particles canbe composed of hexagonal ferrites such as barium ferrites. In order toimprove the required characteristics, the preferred magnetic powder maycontain various additives, such as semi-metal or non-metal elements andtheir salts or oxides such as Al, Co, Y, Ca, Mg, Mn, Na, etc. Theselected magnetic powder may be treated with various auxiliary agentsbefore it is dispersed in the binder system, resulting in the primarymagnetic metal particle pigment. Preferred pigments have an averageparticle length no greater than about 100 nanometers (nm), preferably nomore than about 80 nm. Such pigments are readily commercially availablefrom companies such as Toda Kogyo, KDK, and Dowa Mining Company.

[0027] The use of the higher coercivity pigment at a higher volume alongwith the binders and other ingredients discussed below in the magneticlayer yields a magnetic recording medium exhibiting significantlynarrowed pulsewidth, when measured by recording a signal on a magneticrecording medium at a sufficiently low density that the transitions areisolated from one another; i.e., they do not interact or interfere withone another. The amplified, unequalized and unfiltered signal from theread head is displayed on an oscilloscope and the width along the timeaxis of the resulting positive and/or negative pulses halfway from thebaseline to their peaks is measured. This time interval is multiplied bythe tape transport speed to obtain the pulsewidth, as a distance. ThePW50 for the magnetic recording medium of the invention is less thanabout 500 nm, preferably less than about 450 nm.

[0028] In addition to the preferred primary magnetic metal particlepigment described above, the metal particle pigment of the upper layerfurther includes carbon particles. A small amount, preferably less than2%, of at least one large particle carbon material is also included,preferably a material that includes spherical carbon particles. Thelarge particle carbon materials have a particle size on the order offrom about 50 to about 500 nm, more preferably from about 100 to about300 nm. Spherical large carbon particle materials are known andcommercially available, and in commercial form can include variousadditives such as sulfur to improve performance. The remainder of thecarbon particles present in the upper layer are small carbon particles,i.e., the particles have a particle length on the order of less than 100nm, preferably less than about 75 nm.

[0029] The magnetic upper layer also includes an abrasive or headcleaning agent (HCA) component. One preferred HCA component is aluminumoxide. Other abrasive grains such as silica, ZrO₂, Cr₂O₃, etc., can alsobe employed, either alone or in mixtures with aluminum oxide or eachother.

[0030] The binder system associated with the upper layer preferablyincorporates at least one binder resin, such as a thermoplastic resin,in conjunction with other resin components such as binders andsurfactants used to disperse the HCA, a surfactant (or wetting agent),and one or more hardeners. In one preferred embodiment, the bindersystem of the upper layer includes a combination of a primarypolyurethane resin and a vinyl resin. Examples of polyurethanes includepolyester-polyurethane, polyether-polyurethane,polycarbonate-polyurethane, polyester-polycarbonate-polyurethane, andpolycaprolactone-polyurethane. The vinyl resin is frequently a vinylchloride resin, a vinyl chloride-vinyl acetate copolymer, vinylchloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-vinylacetate-maleic anhydride and the like. Resins such asbis-phenyl-A-epoxy, styrene-acrylonitrile, and nitrocellulose may alsobe acceptable in certain magnetic recording medium formulations.

[0031] In an alternate embodiment, the vinyl resin is a non-halogenatedvinyl copolymer. Useful vinyl copolymers include copolymers of monomerscomprising (meth)acrylonitrile; a nonhalogenated, hydroxyl functionalvinyl monomer; a nonhalogenated vinyl monomer bearing a dispersinggroup, and one or more nonhalogenated nondispersing vinyl monomers. Apreferred nonhalogenated vinyl copolymer is a copolymer of monomerscomprising 5 to 40 parts of (meth)acrylonitrile, 30 to 80 parts of oneor more nonhalogenated, nondispersing, vinyl monomers, 5 to 30 parts byweight of a nonhalogenated hydroxyl functional, vinyl monomer, and 0.25to 10 parts of a nonhalogenated, vinyl monomer bearing a dispersinggroup.

[0032] In one preferred embodiment, the primary polyurethane binder isincorporated into the upper layer in an amount of about 4 to about 10parts by weight, and preferably about 6 to about 8 parts by weight,based on 100 parts by weight of the magnetic upper layer pigment, andthe vinyl or vinyl chloride binder is incorporated in an amount of fromabout 7 to about 15 parts by weight, and preferably from about 10 toabout 12 parts by weight, based on 100 parts by weight of the magneticupper layer pigment.

[0033] The binder system further preferably includes an HCA binder usedto disperse the selected HCA material, such as a polyurethane binder (inconjunction with a pre-dispersed or paste HCA). Alternatively, other HCAbinders compatible with the selected HCA format (e.g., powder HCA) areacceptable.

[0034] The magnetic upper layer may further contain one or morelubricants such as a fatty acid and/or a fatty acid ester. Theincorporated lubricant(s) exist throughout the front-side coating and,importantly, at the surface of the upper layer. The lubricant(s) reducesfriction to maintain smooth contact with low drag, and protects themedia surface from wear. Thus, the lubricant(s) provided in both theupper and lower layers are preferably selected and formulated incombination.

[0035] Preferred fatty acid lubricants include stearic acid that is atleast 90 percent pure. Although technical grade acids and/or acid esterscan also be employed for the lubricant component, incorporation of highpurity lubricant materials ensures robust performance of the resultantmedium. Other acceptable fatty acids include myristic acid, palmiticacid, oleic acid, etc., and their mixtures. The upper layer formulationcan further include a fatty acid ester such as butyl stearate, isopropylstearate, butyl oleate, butyl palmitate, butylmyristate, hexadecylstearate, and oleyl oleate. The fatty acids and fatty acid esters may beemployed singly or in combination.

[0036] In a preferred embodiment, the lubricant is incorporated into theupper layer in an amount of from about 1 to about 10 parts by weight,and preferably from about 1 to about 5 parts by weight, based on 100parts by weight of the magnetic upper layer pigment.

[0037] The binder system may also contain a conventional surfactant orwetting agent. Known surfactants, such as phenylphosphonic acid (PPA),4-nitrobenzoic acid, and various other adducts of sulfuric, sulfonic,phosphoric, phosphonic, and carboxylic acids are acceptable.

[0038] The binder system may also contain a hardening agent such asisocyanate or polyisocyante. In a preferred embodiment, the hardenercomponent is incorporated into the upper layer in an amount of fromabout 2 to about 5 parts by weight, and preferably from about 3 to about4 parts by weight, based on 100 parts by weight of the primary lowerlayer pigment.

[0039] The materials for the upper layer are mixed with the primarypigment and coated atop the lower layer. Useful solvents associated withthe lower layer coating material preferably include cyclohexanone (CHO),with a preferred concentration of from about 5% to about 50%, methylethyl ketone (MEK) preferably having a concentration of from about 30%to about 90%, and toluene (Tol) of concentrations from about 0% to about40%. Alternatively, other ratios can be employed, or even other solventsor solvent combinations including, for example, xylene, tetrahydrofuran,methyl isobutyl ketone, and methyl amyl ketone, are acceptable.

[0040] The Lower Layer

[0041] The lower layer of a dual-layer magnetic tape is essentiallynon-magnetic and typically includes a non-magnetic or soft magneticpowder having a coercivity of less than about 300 Oe and a resin bindersystem. By forming the lower layer to be essentially non-magnetic, theelectromagnetic characteristics of the upper magnetic layer are notadversely affected. However, to the extent that it does not create anyadverse affect, the lower layer may contain a small amount of a magneticpowder.

[0042] The pigment or powder incorporated in the lower layer includes atleast a primary pigment material and conductive carbon black. Theprimary pigment material consists of a particulate material, or“particle” selected from non-magnetic particles such as iron oxides,titanium dioxide, titanium monoxide, alumina, tin oxide, titaniumcarbide, silicon carbide, silicon dioxide, silicon nitride, boronnitride, etc., and soft magnetic particles having a coercivity of lessthan 300 Oe. Optionally these primary pigment materials can be providedin a form coated with carbon, tin, or other electroconductive materialand employed as lower layer pigments. In a preferred embodiment, theprimary lower layer pigment material is a carbon-coated hematitematerial (α-iron oxide), which can be acidic or basic in nature.Preferred alpha-iron oxides are substantially uniform in particle size,or a metal-use starting material that is dehydrated by heating, andannealed to reduce the number of pores. After annealing, the pigment isready for surface treatment, which is typically performed prior tomixing with other layer materials such as carbon black and the like.Alpha-iron oxides are well known and are commercially available fromDowa Mining Company, Toda Kogyo, KDK, Sakai Chemical Industry Co, andothers. The primary pigment preferably has an average particle size ofless than about 0.25 μm, more preferably less than about 0.15 μm.

[0043] Conductive carbon black material provides a certain level ofconductivity so as to prohibit the front coating from charging withstatic electricity and further improves smoothness of the surface of theupper magnetic layer formed thereon. The conductive carbon blackmaterial is preferably of a conventional type and is widely commerciallyavailable. In one preferred embodiment, the conductive carbon blackmaterial has an average particle size of less than about 20 nm, morepreferably about 15 nm. In the case where the primary pigment materialis provided in a form coated with carbon, tin or other electroconductivematerial, the conductive carbon black is added in amounts of from about1 to about 5 parts by weight, more preferably from about 1.5 to about3.5 parts by weight, based on 100 parts by weight of the primary lowerlayer pigment material. In the case where the primary pigment materialis provided without a coating of electroconductive material, theconductive carbon black is added in amounts of from about 5 to about 18parts by weight, more preferably from about 8 to about 12 parts byweight, based on 100 parts by weight of the primary lower layer pigmentmaterial. The total amount of conductive carbon black andelectroconductive coating material in the lower layer is preferablysufficient to provide a resistivity at or below about 1×10¹⁰ ohm/cm².

[0044] The lower layer can also include additional pigment componentssuch as an abrasive or head cleaning agent (HCA). One preferred HCAcomponent is aluminum oxide. Other abrasive grains such as silica, ZrO₂,Cr₂O₃, etc., can be employed.

[0045] The binder system or resin associated with the lower layerpreferably incorporates at least one binder resin, such as athermoplastic resin, in conjunction with other resin components such asbinders and surfactants used to disperse the HCA, a surfactant (orwetting agent), and one or more hardeners. In one preferred embodiment,the binder system of the lower layer includes a combination of a primarypolyurethane resin and a vinyl chloride resin, a vinyl chloride-vinylacetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer,vinyl chloride-vinyl acetate-maleic anhydride, or the like. In analternate embodiment, the vinyl resin is a nonhalogenated vinylcopolymer. Useful vinyl copolymers include copolymers of monomerscomprising (meth)acrylonitrile; a nonhalogenated, hydroxyl functionalvinyl monomer; a nonhalogenated vinyl monomer bearing a dispersinggroup, and one or more nonhalogenated nondispersing vinyl monomers. Apreferred nonhalogenated vinyl copolymer is a copolymer of monomerscomprising 5 to 40 parts of (meth)acrylonitrile, 30 to 80 parts of oneor more nonhalogenated, nondispersing, vinyl monomers, 5 to 30 parts byweight of a nonhalogenated hydroxyl functional, vinyl monomer, and 0.25to 10 parts of a nonhalogenated, vinyl monomer bearing a dispersinggroup.

[0046] Examples of useful polyurethanes include polyester-polyurethane,polyether-polyurethane, polycarbonate-polyurethane,polyester-polycarbonate-polyurethane, and polycaprolactone-polyurethane.Resins such as bisphenol-A epoxide, styrene-acrylonitrile, andnitrocellulose may also be acceptable.

[0047] In a preferred embodiment, a primary polyurethane binder isincorporated into the lower layer in amounts of from about 4 to about 10parts by weight, and preferably from about 6 to about 8 parts by weight,based on 100 parts by weight of the primary lower layer pigment. In apreferred embodiment, the vinyl binder or vinyl chloride binder isincorporated into the lower layer in amounts of from about 7 to about 15parts by weight, and preferably from about 10 to about 12 parts byweight, based on 100 parts by weight of the primary lower layer pigment.

[0048] The binder system further preferably includes an HCA binder usedto disperse the selected HCA material, such as a polyurethane binder (inconjunction with a pre-dispersed or paste HCA). Alternatively, other HCAbinders compatible with the selected HCA format (e.g., powder HCA) areacceptable.

[0049] The binder system may also contain a conventional surfacetreatment agent. Known surface treatment agents, such asphenylphosphonic acid (PPA), 4-nitrobenzoic acid, and various otheradducts of sulfuric, sulfonic, phosphoric, phosphonic, and carboxylicacids are acceptable.

[0050] The binder system may also contain a hardening agent such asisocyanate or polyisocyanate. In a preferred embodiment, the hardenercomponent is incorporated into the lower layer in amounts of from about2 to about 5 parts by weight, and preferably from about 3 to about 4parts by weight, based on 100 parts by weight of the primary lower layerpigment.

[0051] The lower layer may further contain one or more lubricants suchas a fatty acid and/or a fatty acid ester. The incorporated lubricant(s)exist throughout the front-side coating and, importantly, at the surfaceof the upper layer. The lubricant(s) reduces friction to maintain smoothcontact with low drag, and protects the media surface from wear. Thus,the lubricant(s) provided in both the upper and lower layers arepreferably selected and formulated in combination. By way of background,conventional magnetic recording tape formulations employ technical gradefatty acids and fatty acid esters as the lubricant(s). It hassurprisingly been found that these technical grade lubricant materialscontribute to formation of sticky debris in the front coating due tomigration of impurities to the front coating surface. This debris, inturn, can lead to poor tape performance, due to contamination ofrecording heads and other media transport surfaces, interference withlubricity of the medium in transport causing excessive frictional drag,and media wear.

[0052] In a preferred embodiment, the lower layer includes stearic acidthat is at least 90 percent pure as the fatty acid. Although technicalgrade acids and/or acid esters can also be employed for the lubricantcomponent, incorporation of high purity lubricant materials ensuresrobust performance of the resultant medium. Alternatively, otheracceptable fatty acids include myristic acid, palmitic acid, oleic acid,etc., and their mixtures. The lower layer formulation can furtherinclude a fatty acid ester such as butyl stearate, isopropyl stearate,butyl oleate, butyl palmitate, butylmyristate, hexadecyl stearate, andoleyl oleate. The fatty acids and fatty acid esters may be employedsingly or in combination. In a preferred embodiment, the lubricant isincorporated into the lower layer in an amount of from about 1 to about10 parts by weight, and preferably from about 1 to about 5 parts byweight, based on 100 parts by weight of the electroconductive-coatedprimary lower layer pigment.

[0053] The materials for the lower layer are mixed with the surfacetreated primary pigment and the lower layer is coated to the substrate.Useful solvents associated with the lower layer coating materialpreferably include cyclohexanone (CHO), with a preferred concentrationof from about 5% to about 50%, methyl ethyl ketone (MEK) preferablyhaving a concentration of from about 30% to about 90%, and toluene(Tol), of concentrations from about 0% to about 40%. Alternatively,other ratios can be employed, or even other solvents or solventcombinations including, for example, xylene, tetrahydrofuran, methylisobutyl ketone, and methyl amyl ketone, are acceptable.

[0054] Back Coat

[0055] The back coat is generally of a type conventionally employed, andthus primarily consists of a soft (i.e., Moh's hardness<5) non-magneticparticle material such as carbon black or silicon dioxide particles. Inone embodiment, the back coat layer comprises a combination of two kindsof carbon blacks, including a primary, small carbon black component anda secondary, large texture carbon black component, in combination withappropriate binder resins. The primary, small carbon black componentpreferably has an average particle size on the order of from about 10 toabout 25 nm, whereas the secondary, large carbon component preferablyhas an average particle size on the order of from about 50 to about 300nm.

[0056] As is known in the art, back coat pigments dispersed as inks withappropriate binders, surfactant, ancillary particles, and solvents aretypically purchased from a designated supplier. In a preferredembodiment, the back coat binder includes at least one of: apolyurethane polymer, a phenoxy resin, or nitrocellulose added in anamount appropriate to modify coating stiffness as desired.

[0057] Substrate

[0058] The substrate can be any conventional non-magnetic substrateuseful as a magnetic recording medium support. Exemplary substratematerials useful for magnetic recording tapes include polyesters such aspolyethylene terephthalate (PET), polyethylene naphthalate (PEN), amixture of polyethylene terephthalate and polyethylene naphthalate;polyolefins (e.g., polypropylene); cellulose derivatives; polyamides;and polyimides. Preferably, polyethylene terephthalate or polyethylenenaphthalate is employed.

[0059] Although specific embodiments have been described herein forpurposes of description of the preferred embodiment, it will beappreciated by those of ordinary skill in the art that a wide variety ofalternate and/or equivalent implementations calculated to achieve thesame purposes may be substituted for the specific embodiments describedwithout departing from the scope of the present invention. Those withskill in the chemical, mechanical, electromechanical, electrical, andcomputer arts will readily appreciate that the present invention may beimplemented in a very wide variety of embodiments. This application isintended to cover any adaptations or variations of the preferredembodiments discussed herein. Therefore, it is manifestly intended thatthis invention be limited only by the claims and the equivalentsthereof.

EXAMPLES

[0060] The following table lists the physical attributes along with thePW50 results measured at 1.77 m/s. The read head had a constructionsimilar to that of a commercially available Ultrium® LTO 1 head with aread gap shield to shield distance of 0.33 micron.

Comparative Example C1

[0061] Example C1 in Table 1 is a commercially available Ultrium LTO 1dual-layer tape.

Examples 4, 7 and 8

[0062] Examples 4, 7 and 8 in Table 1 are dual-layer tapes comprising amagnetic upper layer and non-magnetic lower layer coated on a 6.0 μm PETsubstrate (Ex. 4), a 4.5 μm PEN substrate (Ex. 7), and a 6.0 μm PENsubstrate (Ex 8), respectively. In addition, each of the tapes has aback coat on the opposite side of the substrate to the magnetic layer.Both the magnetic layer and non-magnetic sublayer use a binder systemcomprising a PVC-vinyl copolymer (MR 104) and a commercially availablepolyurethane (UR-4122) polymer. In addition to the binders, theformulation contains a mixture of fatty acid (stearic acid) and fattyacid esters (butyl stearate and palmitate) as lubricants, alumina as ahead cleaning agent, and carbon particles. The magnetic particles usedin these examples are acicular metal particles with a long axis lengthand coercivity as indicated in Table 1. Magnetic orientation was carriedout in a conventional manner by passing the coated tape through twopairs of complimentary opposing field permanent magnets while themagnetic and sublayer coatings were in the process of drying.

[0063] After drying, the tape was in-line steel-on-compliant calenderedfollowed by off-line steel-on-steel calendering.

Examples 1 and 5

[0064] The tapes used in examples 1 and 5 shown in Table 1 were preparedfrom dispersions and coated similar to those used in Examples 4, 7 and8. The orientation was carried out using a series of inductive coilmagnets such that the magnetic and sublayer coatings were dried to anon-mobile state while in the coil's magnetic field. The tapes werein-line steel-on-steel calendered; no off-line calendering was done. Thetapes were prepared with 6.0 μm PET substrate and the magnetic particleused was the same as that used in Examples 4, 7, and 8.

Examples 2, 3 and 6

[0065] The tapes used in examples 2, 3 and 6 were prepared as describedin Examples 4, 7 and 8 except that after drying, the tapes were in-linesteel-on-compliant calendered followed by off-line steel-on-compliantcalendering. TABLE 1 Exam- MP Length Coercivity Orientation Mr*t PW50ple (nm) (Oe) Ratio (memu/cm²) (nm) 1 75 2640 2.3 1.14 343 2 75 2500 2.11.59 361 3 75 2500 2.1 2.12 366 4 75 2336 2.3 2.84 384 5 75 2548 2.14.42 464 6 75 2500 2.1 3.26 425 7 75 2539 2.2 4.73 484 8 75 2532 2.23.02 452 C1   110 1880 2.3 4.28 544

What is claimed is:
 1. A dual-layer magnetic recording medium comprisinga non-magnetic substrate having a front side and a back side, a lowersupport layer formed over the front side and a magnetic upper recordinglayer formed over said lower layer, comprising a volume concentration ofat least about 35% of a primary magnetic metallic particulate pigmenthaving a coercivity of at least about 2000 Oe, said magnetic pigmentparticles having an average particle length of no more than about 100nm, and a binder for the pigment, wherein said medium has aremanence-thickness product, Mr*t, of less than about 5.0 memu/cm², anorientation ratio greater than about 2.0, and a PW50 of less than about500 nm.
 2. A dual-layer magnetic recording medium according to claim 1wherein said magnetic recording medium has a remanence-thickness productof less than 4.0 memu/cm².
 3. A dual-layer magnetic recording mediumaccording to claim 1 wherein said upper magnetic layer comprises saidmagnetic metallic particulate pigment in a volume concentration of atleast about 40%.
 4. A dual-layer magnetic recording medium according toclaim 1, having an orientation ratio greater than about 2.2.
 5. Adual-layer magnetic recording medium according to claim 1 wherein themagnetic recording medium has a PW50 of less than about 450 nm.
 6. Adual-layer magnetic recording medium according to claim 1 wherein saidmagnetic pigment has a coercivity of at least about 2300 Oe, and themagnetic pigment particles have an average length no greater than about80 nm.
 7. A dual-layer magnetic recording medium according to claim 1wherein said binder system comprises a hard resin component and a softresin component.
 8. A dual-layer magnetic recording medium according toclaim 7 wherein said soft resin component is a polyurethane resin.
 9. Adual-layer magnetic recording medium according to claim 7 wherein saidhard resin component is a vinyl chloride resin.
 10. A dual-layermagnetic recording medium according to claim 7 wherein said hard resincomponent is a nonhalogenated vinyl resin.
 11. A dual-layer magneticrecording medium according to claim 1 wherein said magnetic recordinglayer further comprises a large carbon particle material.
 12. A duallayer magnetic recording medium according to claim 1 wherein the upperlayer comprises a primary ferromagnetic pigment, aluminum oxide, aspherical large particle carbon material, a polyurethane binder, a vinylchloride binder, a hardener, a fatty acid ester lubricant, and a fattyacid lubricant.
 13. A dual-layer magnetic recording medium according toclaim 1 wherein said lower layer comprises: a pigment powder that isessentially nonmagnetic or a soft magnetic powder having a coercivity ofless than about 300 Oe, and a resin binder system therefor.
 14. Adual-layer magnetic recording medium according to claim 13 wherein saidlower layer further includes a fatty acid ester lubricant, a fatty acidlubricant, and a conductive carbon black material dispersed in saidbinder.
 15. A dual-layer magnetic recording medium according to claim 13wherein said conductive carbon black comprises less than about 5 weightpercent of said lower layer.
 16. A dual-layer magnetic recording mediumaccording to claim 1 further comprising a back coat coated on said backside of said substrate.
 17. A dual-layer magnetic recording mediumaccording to claim 16, wherein the back coat includes a carbon blackpigment, a urethane binder, and at least one compound selected fromphenoxy resin and nitrocellulose.
 18. A magnetic recording mediumaccording to claim 16 wherein the back coat further comprises carbonblack, and a metal oxide selected from titanium dioxide, aluminum oxide,and a mixture thereof.